How to increase Harmonic distortion for clean sounds

[Alan0354]

Very interesting. So you are saying if one side of the wave gets stretched you get harmonics, but then if the other side gets stretched the same way in the next stage they then cancel? It seems like it would have to be near perfect circumstances to achieve the canceling.. Same component setup, but also same input voltage swing. Also if there was a tone stack between them it seems like the canceling would not occur. This leads me to wonder if the greatest amount of harmonics could be achieved by leaving one side of the wave as linear as possible.

I never said the second stage cancel all the harmonics. Please read my posts again. All I am saying is "in the first pass" it cancel a lot of the asymmetrical shape( which is mainly 2nd harmonic). I am just saying stretch and compress. I did not even get into the IMD. AND I did said I don't talk about the IMD sideband. It just happened that KG posted the article and I agree with that.

Of cause when you mix two stages, you have all the IMD which is shown in the article by KG and those do not get cancel out. Most people here are designer, I assume everyone here is familiar with IMD. , this is so basic and essential in amplifier design!!! I made it very specific about stretch and compress which is about symmetry, not cancelling all distortion.

Please read post #74.

[eschertron]

First of all, I don't mean to offend anyone.

Alan, don't take my post as an attack. I am trying to validate Tahoebrian's desire for understanding. We should all rely first on our senses, then our textbooks, then... the internet

[Alan0354]

Alan, don't take my post as an attack. I am trying to validate Tahoebrian's desire for understanding. We should all rely first on our senses, then our textbooks, then... the internet

No, I never think it is an attack, I was just trying to explain myself that I never meant to say two cascade stage can cancel all harmonics that everyone seems to think I am saying.

The OP asked how to introduce harmonics for clean sound. We start out talking about how to manipulate the operating point to get the most variation of rp of the triode and plate resistor. The most obvious effect comes out is the asymmetrical wave form ( 2nd harmonics). And two identical stages of triode tends to cancel( say reduce) the 2nd harmonics and make the wave more symmetrical again. But two cascade stage does not cancel all distortion it add more IMD as the article showed by KG.

[kg]

If you define distortion as creation of side band and IMD, then yes, distortion is distortion. But I look at it in the sound and the source of distortion. OP asked about introducing harmonic in clean sound, that's what we are talking about. Your article on compensating of 2nd harmonics are all on the "clean" sound. When signal starts to clip, it will sound totally different, it start to sound fuzzy. To me, the result sound is totally different, and the mechanism of distortion is totally different. I choose to look at it as separate cases.

you are perfectly free to see it as totally different cases. however, as far as distortion metrics are concerned, they are points along a continuum. when hard clipping occurs, distortion rises rapidly, but all that has happened is that higher order artifacts have suddenly been introduced.

Clipping is signal run into the limit of each rail, "clean" distortion is about changing the internal plate resistance of the triode.

what is the rp of a tube that has no plate current?

This is getting into definition of distortion and I really not interested in what is right or wrong.

that's unfortunate, because until we can have a meeting of the minds as to definitions of terms, i don't see that we're going to have a meaningful conversation.

[Dave H]

Vox AC301990:http://www.webphix.com/schematic%20h...s/ac301990.pdf

V4 through 0.047uF and volume pot, through RC network to V7. That's two stages.

Vox AC1978:http://www.webphix.com/schematic%20h...s/ac301978.pdf

V4B through 0.047uF and volume pot, through 470K and RC network to V12B. Two stages.

You don't consider clean as when you are at the verge of breaking up, you are in the soft clipping mode. That's the reason the odd harmonics is so high. BTW, you can artificially raise the even harmonics by soft clipping if you intentionally do asymmetrical clipping by biasing the plate close to cut off or bottoming out. I would say at least lower the input voltage by half to one quarter.

Right, those are reverb AC30s. I’ve never seen one of those. I was looking at the standard model schematics. AC30s don't need reverb

Below are the spectrum plots for my ‘AC15’ at 4W and 8W output. I also measured the distortion at the first stage and PI outputs but they are HiFi by guitar amp standards. I don’t think the pre-amp distortion would be audible so all the goodness must be happening in the power amp even when it’s well below clipping.

J20-15 Spectrum 4W.pdf
J20-15 Spectrum 8W.pdf

[Alan0354]

Right, those are reverb AC30s. I’ve never seen one of those. I was looking at the standard model schematics. AC30s don't need reverb

Below are the spectrum plots for my ‘AC15’ at 4W and 8W output. I also measured the distortion at the first stage and PI outputs but they are HiFi by guitar amp standards. I don’t think the pre-amp distortion would be audible so all the goodness must be happening in the power amp even when it’s well below clipping.

[ATTACH]27158[/ATTACH]
[ATTACH]27159[/ATTACH]

Is the freq 500Hz? Yes, the distortion is quite low. One plot the fundamental is +18dB and the 2nd is around -25dB and the 3rd is slightly higher.

Notice, at least in the schematics I have, the plate resistors are 220K. From my example at the beginning of this thread, seem the distortion gets reduced using a plate resistor much higher than the internal plate resistor. Now I am not making a statement, I am just guessing. Please don't take it as if I insist this is correct.

I think this thread is very educational.

[kg]

From my example at the beginning of this thread, seem the distortion gets reduced using a plate resistor much higher than the internal plate resistor. Now I am not making a statement, I am just guessing. Please don't take it as if I insist this is correct.

no, it is absolutely correct: triodes love working their plates into current sources. take even the mundane 12ax7 plate curves , and draw a horizontal load line at say 1mA. by my eye the vp crosses 39, 88, 137, 188, 237, 288, 340, 390, 438 for each 0.5vg increment. that's almost EXACTLY 50v/0.5v.

lo and behold, the mu of the tube is listed as 100.

[Chuck H]

And the actual gain of a 12ax7 triode with a 68k plate resistor and a fully bypassed cathode is indeed 50. (I think 68k is the typical plate resistance of a 12ax7, so effectively forming a 50/50 voltage divider)

What's interesting is the greater distortion resulting from this symmetry. Could this be because the current is being equally shared between the resistances generating more 2nd harmonic?

[exclamationmark]

An interesting way to add some more second harmonic distortion is by taking your typical interstage divider and putting a diode across the top half of the divider. Depending on the polarity of the diode, one half of the wave will be un-attenuated (only being dropped by the diode forward voltage drop) and the other half of the wave will be attenuated by the interstage divider. I can't exactly remember which amplifier I saw this in, but I think it was a bogner.

As shown by the scope photos, a chimy "clean" sound is rarely clean - you will usually see significant rounding or clipping of the waveform. Hence (IMO) the actual THD contributed by the transfer characteristics of the tube in the non-clipping areas (ie, the area where the tube doesn't draw significant grid current or slam into the voltage rail), makes a relatively small difference in the grand scheme of things, compared to the effect the bias/load line has on the overdrive characteristics.

[exclamationmark]

Edit: Whoops, double post!

[Jazz P Bass]

How about a way to 'see' the harmonic distortion.

A dual trace scope would be ideal.

Set one trace at the input & the other at whatever tube output we would be interested in.

Nice, slightly rounded wave form peaks would be the onset of 'distortion'.

The extreme would look like a single ended output. (top & bottom are also different widths.)

[Gingertube]

Example front end that I have used for increased harmonic distortion - This is a Trainwreck Express with the input stage modified.
I run 2 triodes sections in parallel but use separate cathode bias resistors and bypass caps to run the two triodes at different operating points. Note that one triode section is "standard" values, the other is low current with some high frequency emphasis from a smaller bypass cap, to add some harmonic content to the sound.

This is consistent with what I said in other parts of this thread.

Cheers,
Ian

[Alan0354]

Example front end that I have used for increased harmonic distortion - This is a Trainwreck Express with the input stage modified.
I run 2 triodes sections in parallel but use separate cathode bias resistors and bypass caps to run the two triodes at different operating points. Note that one triode section is "standard" values, the other is low current with some high frequency emphasis from a smaller bypass cap, to add some harmonic content to the sound.

This is consistent with what I said in other parts of this thread.

Cheers,
Ian

How is the added stage that emphasis on highs change the sound?

[Tahoebrian5]

That is pretty interesting. Another vote for how it sounds or even a clip would be fun